Work out desiccant quantity instantly with clear inputs, formula shown and shareable results.
Desiccant must absorb both the moisture sealed inside with the air and the moisture permeating through the pack over the protection period. Silica gel adsorbs roughly 20% of its own weight at moderate humidity, so the mass required is total water divided by that capacity.
Moisture load
Ingress = WVTR x pack area x days; enclosed air moisture = volume (L) x 0.0173 g/L at 23 C, 60% RH
Desiccant mass
Desiccant = total water / adsorption capacity (0.2 for silica gel)
Molecular sieve holds more at low humidity and is better for very dry requirements; clay is cheaper for less demanding uses; calcium oxide is used for very long-term protection. Match the isotherm to the target humidity.
For a small pack it can dominate. One litre of air at 23 C and 60% RH carries about 12 mg of water, which is significant when the total budget is a few hundred milligrams.